docs: update Middleware API reference (#30485)
docs: update Middleware API reference (#30485)

I changed the order of `NextRequest` and `NextFetchEvent` as they are presented in the function signature.

There also seemed to be some issues with the heading levels.

Also, `NextResponse` is a documented API, but the example code uses `Response`. I wasn't sure if this was a mistake in the docs or in the code.

I noticed that other similar docs pages have a collapsible `Examples` section at their top:

- https://nextjs.org/docs/api-reference/next/image
- https://nextjs.org/docs/api-reference/next/head
- https://nextjs.org/docs/api-reference/next/amp

Image below:
![image](https://user-images.githubusercontent.com/18369201/139145600-9f45d5be-b11c-4780-be95-c7f2ebeb49a9.png)

Should the Middleware docs have the same?
zeit/next.js,JeromeFitz/next.js,JeromeFitz/next.js,azukaru/next.js,JeromeFitz/next.js,azukaru/next.js,JeromeFitz/next.js,zeit/next.js,zeit/next.js,azukaru/next.js,azukaru/next.js